WASHINGTON (MarketWatch) - The underlying rate of U.S. inflation decelerated in March, the Labor Department said Tuesday. The consumer price index decreased 0.2%, driven by a 4.4% drop in energy prices, the biggest drop in energy prices since November. Food prices were flat. The only big jump came in process for used cars. The core CPI, which excludes food and energy costs, rose 0.1% in March. Economists were expecting the CPI to fall 0.1% in March after jumping 0.7% in the prior month. The core rate was expected to rise 0.2% after rising 0.2% in the previous month.